Reaction turbines are a type of hydraulic turbine that use the reaction force of water flow to generate power. Unlike impulse turbines (like the Pelton turbine) which convert the water's pressure energy entirely into kinetic energy before it hits the runner, reaction turbines utilize both the pressure energy and kinetic energy of the water flowing through the runner.

Understanding Reaction Turbines and Their Application

The primary characteristic that distinguishes reaction turbines from impulse turbines is how they extract energy from the water. In a reaction turbine, the runner is fully submerged in water, and pressure changes occur as water flows through the runner blades. This pressure change, along with the change in kinetic energy, drives the turbine.

Different types of reaction turbines are suited for different ranges of head (the vertical height difference from the water surface to the turbine) and discharge (the volume flow rate of water):

• Francis Turbines: These are mixed-flow reaction turbines suitable for medium heads and medium discharges.
• Kaplan Turbines: These are axial-flow reaction turbines designed for low heads but very high discharges.

Considering the range covered by different types of reaction turbines, they are generally applied where the available water head is relatively low compared to impulse turbines, but the volume of water flowing (discharge) is significant.

Analyzing the Options

Let's look at the given options in the context of reaction turbine applications:

• Low head: While reaction turbines can operate at low heads, this option is too general as it doesn't specify the discharge condition.
• High head: High head applications are typically suited for impulse turbines like the Pelton turbine, not reaction turbines.
• High head and low discharge: This combination is characteristic of impulse turbines (e.g., Pelton turbine), not reaction turbines.
• Low head and high discharge: This combination perfectly describes the conditions where reaction turbines, especially Kaplan turbines, are most effectively used. Francis turbines also fall within the reaction category and cover medium heads and higher discharges than impulse turbines. Therefore, this option represents the typical application range for reaction turbines as a class, particularly focusing on their suitability for higher flows at lower heads compared to impulse types.

Based on the operational principles and typical applications of various types of reaction turbines like Francis and Kaplan turbines, they are most commonly used for conditions characterized by a low available head but a high volume of water flow.